The present invention relates to a method for charging rechargeable lithium accumulators where a charging current is injected into the accumulator and the voltage is monitored on the accumulator 10 during the injection process.
The invention further relates to a charging device for charging rechargeable lithium accumulators having first means for injecting a charging current into the accumulator and second means for monitoring the voltage on the accumulator during the injection process.
Finally, the invention relates to a lithium accumulator having first means for connecting the accumulator to a charging device and second means for monitoring the voltage on the accumulator during injection of a charging current.
A method, a charging device and a lithium accumulator of the afore-mentioned kind are known from DE 198 38 137 A1.
In mobile applications, there are in use today accumulators of different kinds, known as “battery packs”, for example nickel-cadmium batteries (NiCd), nickel-metal hydride batteries (NiMH) and lithium-ion batteries. In the latter case, batteries using different metals (Mn, Ni, Co, Ti) in diverse combinations are known. While LiCoO2 batteries are in use for electronic devices, such as portable computers, LiMn2O4, LiNiO2, LiTi5O12, LiNi1-xCoxO2 and the like are used for applications in electric tools, which is the field of particular interest for purposes of the present invention. Those accumulators have a cell mains voltage of between 3.6 and 3.7 Volts. More recently, lithium ion batteries of the lithium iron phosphate type have become known that have a cell mains voltage of 3.3 Volts.
It is a general problem of rechargeable accumulators that overcharging has to be avoided during the recharging process. The accumulators listed above show different degrees of sensitivity to overcharging, depending on type.
Lithium-ion accumulators are very sensitive to overcharging and may even catch fire or explode in extreme cases under overcharging conditions. On the other hand, it is desirable, with a view to achieving the longest possible operating period for the devices to be operated using those accumulators, that the accumulator be charged as completely as possible, that means that its capacity be utilized to the fullest possible degree.
Similarly, accumulators have different degrees of sensitivity to complete discharging, which even may lead to destruction of the accumulator with some accumulator types.
With a view to protecting accumulators from being damaged during charging and discharging, one has developed many criteria aimed at preventing overcharging and complete discharging by suitable control of the current supply. For example, it is a necessity that the number of cells and the charging parameters (final charging voltage) be known. In practice, these criteria often can be met with difficulty only. On the other hand, given the fact that accumulators are increasingly regarded as low-end articles, the target conflict between effective protection of the accumulator on the one hand and the related cost on the other hand has to be solved.
In the context of the present invention, the interest concentrates on lithium accumulators which, as has been mentioned before, are sensitive to overcharging. While for obvious reasons the method known from the afore-mentioned DE 198 38 137 A1, envisaged for the special case where an accumulator of an implant located in a human body is to be charged transcutaneously, is safe in operation with respect to the accumulator it does not make full use of the potential capacity of the accumulator.